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How to work out your swim zones

The best test for swimming threshold pace is the CSS (Critical Swim Speed) test:


Start by warming up for 200m easy relaxed stroke
Then complete 1 x 400 FS maximal effort time trial and record the time,
Take 5 mins easy active recovery,
Then complete 1 x 200 FS maximal effort time trial and record the time,
Cool Down with 100 choice stroke.

From there, you can calculate your CSS pace using this online calculator here.


This gives you your lactate threshold pace and can be a useful tool in structuring your training sessions to ensure you are getting the benefit out of every session.


Next, you will need to calculate your training zones for swimming which you can add into Training Peaks and have as a useful guide for intensity targets in the training sessions. You can do that through the table opposite

So how does CSS work?


For distance swimmers - including open water athletes and triathletes - one physiological factor is all important: your lactate threshold. If you can improve your speed at threshold then your race times will drop.

Compared to traditional master's swim sets, CSS training involves swimming at a slightly (only slightly!) slower pace but with much shorter recoveries between each swim. This keeps things focused on developing your aerobic system, which is what you need to become a better distance swimmer.


The problem with sprinting hard and then recovering is that it focuses much more on your anaerobic system, which is great for sprinters but far from ideal for distance swimmers and triathletes.


Now you know your CSS speed you can use it to set a pace for your quality swim sets. Remember, target CSS speed to develop your lactate threshold and become a faster distance swimmer.


At the bottom of the page are some examples - these are challenging sets you might swim once a week for the main set in your quality swim. The key point with all these sets is sustained speed with short recoveries. These are just examples, make up your own sessions to keep things interesting.


Compared to how you normally train you might find the pace slightly slower but the recoveries much shorter. It's a different sort of challenge.


CSS feels easy for the first few hundred metres but creeps up on you as the sets go on. The word 'relentless' probably describes it best!

Please note that some values will be above 0.59, so all you need to do in those instances is add an extra minute plus the remaining seconds.


So for example 1.75 = 1 min 75 secs = 2.15min/100m, and 1.95 =1 min 95 secs = 2.35min/100m


Your target zone ranges will then be 3 secs below and 2 secs above the values calculated here.

Pacing Skills & Tempo Trainers


When you swim CSS sets it's important to pace things well, accurately targeting CSS pace (in most cases). If you start too fast and blow-up it will hurt more and you won't get the same fitness gains for your efforts.


Try and swim each repetition at the same pace. Very often this will mean the first few hundred metres feels fairly steady - good pacing technique is an important skill to learn and is one of the key differences between amateur and elite swimmers.


To help develop this important skill you could also use a Finis Tempo Trainer Pro. This is a small beeper you place under your swim cap when you swim. You set the beep for a given interval (say every 30 seconds), wait for the beep to come and then immediately start your swim.


You then pace your swim so that you turn and push-off each time it beeps. Get ahead of the beeper and you know you are going too fast and risk blowing up later in the set.

Find out more about them here: products/finis-tempo-trainer-pro


The Mental Side

If you are used to doing swim sets with more recovery, a CSS session can look pretty nasty when you write it down. You've got to sustain a fast pace for quite a while and you don't get much recovery time between swims.


Most of this is just a mental thing because you're not used to swimming like that. Swimming 4x 400m fast sounds tough but the running equivalent in terms of time is 4x 1 mile, in terms of time. Runners, doesn't sound so tough?


We recommend you don't think too much, just do it! It sounds much harder than it is.


What if it really is too hard, or it feels too easy?

The CSS test isn't foolproof.

If it feels too easy to complete the example sessions above then it is too easy. Increase the pace slightly until it feels hard but do-able. Just make sure you sustain the same pace throughout - don't fade.

Alternatively, if it really is too hard and you can't complete a decent set then swallow your pride and back off the pace a couple of seconds per 100m. That doesn't sound like much but it can make all the difference

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